Interrogation of multidrug resistance (MDR1) P-glycoprotein (ABCB1) expression in human pancreatic carcinoma cells: Correlation of ^99mTc-Sestamibi uptake with western blot analysis

Histopathological studies indicate that ∼63% of pancreatic tumors express multidrug resistance (MDR1) P-glycoprotein (Pgp) and its polymorphic variants. However, Pgp expression detected at the mRNA or protein level does not always correlate with functional transport activity. Because Pgp transport activity is affected by specific mutations and the phosphorylation state of the protein, altered or less active forms of Pgp may also be detected by PCR or immunohistochemistry, which do not accurately reflect the status of tumor cell resistance. To interrogate the status of the functional expression of MDR1 Pgp in MiaPaCa-2 and PANC-1 cells, cellular transport studies using ^99mTc-Sestamibi were performed and correlated with western blot analysis. Biochemical transport assays in human pancreatic carcinoma MiaPaCa-2 and PANC-1 cells, human epidermal carcinoma drug-sensitive KB-3-1 cells, and human breast carcinoma MCF-7 cells (negative controls), and human epidermal carcinoma drug-resistant KB-8-5 cells, human breast carcinoma stably transfected with Pgp MCF-7/MDR1Pgp cells, and liver carcinoma HepG2 cells (positive controls) were performed. Protein levels were determined using a monoclonal antibody C219. ^99mTc-Sestamibi demonstrates accumulation in human pancreatic carcinoma MiaPaCa-2 and PANC-1 cells. Uptake profiles are not affected by treatment with LY335979, a Pgp inhibitor, and correlate with western blot analysis. These cellular transport studies indicate an absence of Pgp at a functional level in MiaPaCa-2 and PANC-1 cells. Because major pancreatic tumors originate from the pancreatic duct and ^99mTc-Sestamibi undergoes a dominant hepatobiliary mode of excretion, it would not be a sensitive probe for imaging pancreatic adenocarcinomas. Following interrogation of the functional status of Pgp in other pancreatic carcinoma cells, chemotherapeutic drugs that are also MDR1 substrates could offer alternative therapeutics for treating pancreatic adenocarcinomas.